Antennas have been used for various usages of transmitting or receiving radio waves. Particularly, a monopole antenna is used as one of the antennas which have relatively strong radiation patterns with respect to a direction parallel to an installation surface of the antenna. However, depending on a case, an available space for installing an antenna may be limited, so that it is difficult to install a monopole antenna. Accordingly, a monopole antenna which may be manufactured in a compact size, has been suggested (see, e.g., Japanese Laid-Open Patent Publication No. 2001-308630). The monopole antenna includes a grounding conductor, a power feeding unit positioned on a surface of the grounding conductor, an antenna element connected to the power feeding unit, a lateral conductor which surrounds a region around a space including the antenna element while being spaced apart from the antenna element, and a ceiling conductor which faces the grounding conductor with the antenna element interposed therebetween.
According to a grounding environment, there may be a case where a region around the antenna is surrounded by a conductor. In this situation, there may be a case where a radiation characteristic of the antenna does not satisfy a desired characteristic by an influence of the conductor around the antenna. Accordingly, there is suggested a monopole antenna which is designed on the premise that the mono pole antenna is embedded in a manhole cover and includes a circular patch in which power is fed from a center thereof, and two short-circuit strips (see e.g., Non-Patent Document 1).
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2001-308630 and Eun-Suk Yang et al., “A Low Profile Circular Patch Loaded Monopole Antenna Embedded in a Manhole Cover” (Non-Patent Document 1).
In the monopole antenna described in Non-Patent Document 1, in order to obtain a desired radiation characteristic, the manhole cover and the patch are required to be disposed while having a space in some degree, so that there is a need for an installation space to have a size in some degree. However, depending on a case, a gap may not be sufficiently secured between the peripheral conductor and the patch.